Well Cementing Technology for Complex Reservoirs in the Bayan Hetao Basin

The reservoir formations in the Baran Hetao Basin have the characteristics of low maturity, high permeability, low strength and high shale content. The formations drilled have high water sensibility, instability and leaking potential. Deep well cementing is faced with many difficulties, such as long cementing section, low displacement efficiency, low bonding strengths of the two interfaces and high requirements on the mechanical performance of the cement sheath in late fracturing jobs, etc. To solve these problems, a batch-mixing batch-injecting DRJ-2S flushing spacer and a slight-expansion high strength rough cement slurry DRE have been developed with two key cementing materials, which are an interface strengthening agent DRJ-2S and an expanding toughening agent DRE-4S. Laboratory studies show that the DRJ-2S flushing space has a flushing efficiency of more than 96%, which is 30% higher than that of clean water, and a spacer of density 1.60 g/cm³ has 7 d compressive strength of 9.6 MPa. The DRE-2S flushing spacer has the ability of remarkably enhancing the compressive strength of mixed fluids and the bonding strengths of the interfaces of a cement sheath. At 90 ℃ and 21 MPa, a high strength tough slight-expansion cement slurry treated with 4% – 6% DRE-4S has compressive strength of at least 24 MPa, elastic modulus of equal to or less than 7 GPa, and linear ratio of expansion of at least 0.02%, indicating that the cement slurry has good adaptability to mechanical alternation. The DRE cement slurry and the DRJ-2S spacer have been used on more than 40 wells in the Bayan Hetao basin, with average well cementing pass rate of 96.5% or higher, greatly improving the quality of well cementing job in the basin. The application of the DRE and the DRJ-2S has provided a powerful technical support for the safe and efficient development of the complex oil and gas wells, and is worth promoting.